1. Technical Field
The present invention relates to the removal of multivalent ions such as chromium ions from solution. The present invention also permits removal of additional metal ions species which may be present with the multivalent ions.
2. Description of the Prior Art
There is a considerable and growing concern over pollution of the nation's waterways with various contaminants, such as the heavy metals, e.g., chromium, copper, nickel, zinc, mercury, and cadmium. Many of these pollutants enter the nation's waters from industrial sources such as metal finishing or plating plants and from mining sources. Environmental legislation and regulations on the federal, state and local government levels have set forth maximum allowable concentrations of these contaminants which may be discharged into public waters. A present need exists for an economical, yet effective process for treating such waste waters and removing a substantial portion of the contaminants. This is especially critical for plating plant rinse water which customarily contains more than one of the aforementioned metals.
Aqueous plating solutions containing chromium ions are of particular concern. The major portion of chromium in such solutions exists as hexavalent chromium (Cr.sup.+6) which is considered to be harzardous and which has long been recognized as a major stream pollutant.
Certain waste water streams typically contain one or more metals in metal ion concentrations less than 1,000 parts per million, often about 500 parts per million or less. The lower the concentration of the metal ion in the stream, the more difficult it is to recover the metal economically. Streams with concentrations less than about 1,000 parts per million are generally referred to as "dilute aqueous metal bearing streams" or "problem streams". There exists a need for equipment that can handle economically the so-called "problem streams" as well as streams containing higher concentrations of metal ions.
The removal of chromium from chrome-containing waste water by precipitation of chromate hydroxide is known in the art. By way of example, prior U.S. Pat. No. 4,260,491 discloses a process for the removal of chrome from waste water, wherein the chromium is in the hexavalent state, by treating the waste water with (1) a reducing agent suitable for converting hexavalent chromium to trivalent chrome and (2) a magnesium, calcium, ferric or aluminum sulfate or chloride salt to promote the precipitation of chromic hydroxide. The problem with this and other processes in which the end product is a chrome (III) hydroxide precipitate is that suitable landfill areas for chromium hydroxide sludge materials are becoming increasingly scarce and increasingly costly. In addition, this disposal of chromium values represents a substantial loss of a valuable material.
Prior U.S. Pat. No. 3,481,851 discloses a process for reconditioning a used chromic acid containing metal treating solution. The used solution contains trivalent chromium. The used solution is introduced as the anolyte solution into an anode compartment of an electrodialysis cell. An acid catholyte solution is introduced into the cathode compartment. A cation permeable membrane separates the two compartments. On energizing the cell, dissolved foreign ions such as copper, iron, zinc, nickel and cadmium selectively pass through the membrane into the catholyte solution and plate onto the cathode. The chromium ions, present as anion complexes, are restricted from passing through the membrane. In the anolyte, the trivalent chrome reoxidizes at the anode into hexavalent chromium. The process is designed to produce hexavalent chromium. The process is not concerned with removal of chromium ions from a spent solution.
U.S. Pat. No. 3,761,369 shows a somewhat similar technique for reclaiming spent etching fluids containing chromium values and additional extraneous metals such as copper. The technique is carried out in two stages in which, in the first stage, the hexavalent chromium is reduced to the trivalent chromium. Following the reduction of the hexavalent chromium, copper metal is plated from the solution onto a cell cathode until the solution is copper free. The solution containing the trivalent chromium values is then transferred to the cell anode chamber where the trivalent chromium is reoxidized to hexavalent chromium. The process also is not concerned with economically recovering chromium ions from a spent electrolyte solution.
A process similar to that of U.S. Pat. No. 3,481,851 is disclosed in U.S. Pat. No. 4,337,129. This patent also discloses an electrolytic cell for regenerating a solution containing trivalent chromium ions, as well as removing extraneous metals such as copper. In this cell, a cation permeable membrane separates the cell into an anolyte chamber containing an anode and a catholyte chamber containing a cathode. The anolyte chamber contains the spent solution. In the process of electrolysis, employing a direct current, cations migrate through the membrane from the anolyte chamber to the catholyte chamber with the extraneous metal plating out on the cathode. The trivalent chromium ions in the anolyte are reoxidized to hexavalent chromium. As with prior U.S. Pat. No. 3,481,851, this patent does not disclose a process for the removal of multivalent ions, as such term is used herein, such as chromium from an electrolyte solution.
Prior U.S. Pat. No. 4,436,601 discloses a process for electrolytically reducing the concentration of metal ions in a waste water stream. The process uses a cell which contains a plurality of anodes and cathodes arranged in alternating sequence in the cell. The anodes are provided with openings therein and the cathodes are in the form of metalized organic polymer foam reticulates. Waste water is circulated through the cell, and metal sought to be removed is cathodically deposited. The process of this patent, however, is only useful for waste steams containing selected metal ions. In the case of streams which contain a multivalent metal ion such as chromium, the ions tend to oxidize at the anodes and be reduced at the cathodes so that no electrolytic removal of the metal takes place. Specifically in the case of chromium, Cr.sup.+3 is oxidized to Cr.sup.+6 at the anodes. The reverse reaction takes place at the cathodes. Not only does this impede removal of chromium, but it also hinders plating of whatever selected others metals are contained in the stream.